A large number of peptide antigens displayed on tumor cells in the context of class I MHC molecules and recognized by tumor-reactive CD8 T cells have been identified in the last 15 years. However, only a small number are derived from proteins known to be involved in processes that are altered during cellular transformation or metastasis. Antigens of this type are particularly appealing as immunotherapeutic targets for cancer control, since alteration of their structure or expression as a means of immune escape may compromise one or more aspects of the malignant phenotype. Since the last renewal, we have developed a unique mass spectrometry approach to identify and sequence class I MHC associated peptides modified by phosphorylation. This modification is associated with a variety of cellular control processes, some of which are known to be dysregulated in cancer cells. The source proteins for the class I MHC-associated phosphopeptides identified to date are in large part known phosphoproteins. This is consistent with the hypothesis that these peptides are processed during the degradation of folded proteins that are participating in signaling pathways. Using a differential display mass spectrometry approach, we have also established that a subset of these phosphopeptides are selectively displayed on melanoma and ovarian carcinoma cell lines in the context of HLA-A*0201. Most members of this latter group are derived from source proteins that have been shown to be dysregulated in some cancer cells, and to contribute to either cellular transformation or metastasis. Finally, we have shows that several of these phosphopeptides are immunogenic in HLA-A2 transgenic mice and in in vitro human PBMC cultures. In the present application, we propose to expand these efforts in two directions. First, we propose to more directly establish the relationship between the phosphopeptides displayed and: a) signal transduction pathways that are known to be specifically dysregulated in melanoma cells; b) growth and viability of melanoma cell lines; and c) stage and grade of melanoma. Second, we will establish the immunological utility of the phosphopeptides, based on their ability to induce T cells from humans and human class I MHC transgenic mice that can specifically recognize the peptide displayed on tumor cell lines after endogenous processing and presentation. Using selected phosphopeptides, we will explore structural modifications to enhance their immunogenicity while preserving the specificity of the resulting T cells. These two research directions will culminate in the identification of a phosphopeptide that will be utilized as an immunogen in a phase I clinical trial. This work will take advantage of important, and in some cases, unique strengths of the research team in immunology, mass spectrometry, signal transduction research, and experience with peptide-based vaccine trials. Collectively, the studies proposed will characterize a novel set of antigens and assess their potential utility for cancer immunotherapy. Cancer therapies based on stimulating the patient's immune system represent an important new treatment modality. The investigators have identified a group of antigens, the targets of immune responses, which are selectively expressed on human melanoma cells and also potentially related to the control processes that become altered in cancer cells. The goal of this proposal is to establish this relationship more directly, and to test one of these antigens for its ability to stimulate a cancer specific immune response in melanoma patients.